Mars for the Many

Pebbles in Mars Riverbed Traveled for Miles

Mars Riverbed Gives up its Secrets

​The evidence is clear...

Mars was once awash with water. ​Images beamed back to Earth from the Mars Curiosity rover in 2012 contain compelling evidence that, not only was there a lot of water, but that it flowed freely and rapidly.

In 2012 the Mars Curiosity rover beamed images back to Earth containing some of the most concrete evidence that water once flowed in abundance on the planet. Small, remarkably round and smooth pebbles suggested that an ancient riverbed had once carried these rocks and abraded them as they traveled, much like rivers do here on Earth.

To Douglas Jerolmack, a geophysicist at the University of Pennsylvania, and his collaborator Gábor Domokos, a mathematician at Budapest University of Technology and Economics, Curiosity's findings raised a fundamental geological question: Can we use shape alone to interpret the transport history of river pebbles -- on Mars, Earth or any planet?

As Jerolmack said:

Thousands of years ago, Aristotle pondered the question of pebbles on the beach and how they become rounded. But until recently, descriptions of pebble shape have been qualitative, and we lacked a basic understanding of the rounding process.

No longer.

In a new report in Nature Communications, Jerolmack, Domokos and some additional colleagues reported the first-ever way to estimate the distance river pebbles may have traveled -- from their shape alone.

Their findings?

The pebbles in the Mars riverbed traveled roughly 30 miles from their source.

That's pretty compelling evidence that Mars once had an extensive river system. Which would help explain the networks of valleys found all over the planet.

Said Domokos:

An object's shape can itself tell you a lot. If you go to the beach, natural history is written underneath your feet. We started to understand that there is a code that you can read to begin to understand that history.

Rocks flowing in rivers get their shape from abrasion -- banging against other rocks in the mars riverbed and slowly getting smoother and rounder. Because of this action, traditional geophysical models can link a pebble's history to the mass it loses due to collisions with other pebbles. And from the mass lost, you can determine how far the pebble may have traveled.

But mass data is not available for pebbles in a Mars riverbed. So the researchers set the ambitious goal of determining the lost mass of a pebble solely based on its current shape.

As Jerolmack said:​

When you land a multi-billion dollar rover on Mars, you want to extra as much information from the data as possible.

Domokos' work showed that a pebble's shape and mass loss is a purely geometrical problem, regardless of the rock's material or the environment in which it is moving. To prove it, the research team tested the theory in a lab -- rolling limestone fragments in a drum and periodically pausing to record their shape changes and mass loss. The pattern of the rocks' shape change closely followed the curve established by the mathematical theory.

Next the researchers went to a mountain river in Puerto Rico. According to Jerolmack:

We started at the headwaters, where chunks of angular rock are breaking off from the walls of the stream, and went downstream. Every few hundred meters we would pull thousands of rocks out and take images of their silhouette and record their weight.

As an additional confirmation, they performed a similar analysis on rocks at the mouth of a canyon in New Mexico, an environment that more closely mirrors the location where round pebbles were found on Mars.

They tested their method on Earth, in the Mars-like environment of a canyon's mouth in New Mexico.

Plotting the data, they again found a trend between shape evolution and mass loss that agreed with the geometric model Domokos had developed.

But can you use those findings for a Mars riverbed?

Using publicly available images of rounded pebbles on Mars from the Curiosity rover mission, Szabó traced their contours and performed an analysis based on the research team's models. The results suggested that the pebbles had lost approximately 20 percent of their volume.

Applying these calculations to the basalt material found on Mars, with a correction that factored in the reduced Martian gravity, they arrived at the calculation that the pebbles had traveled an estimated 50 kilometers, or about 30 miles from their source. The distance meshed well with what Grotzinger and the Curiosity team had suspected about the pebbles' origin, based on other analyses of the rock's composition and clues as to the direction of water flow, that they were sourced from a crater rim located approximately 30 kilometers away.

Jerolmack noted that the study is not only exciting for what it implies about Mars, but for geophysics here on Earth and even other planetary bodies.

Now we have a new tool we can use to help reconstruct ancient environments on Earth, Mars and other planetary bodies where rivers are found such as Titan.